Recently, new heat-insulation materials for energy saving have been sought. There are heat-insulation materials called aerogels in which silica particles are connected with one another. The silica aerogels are quite different from urethane foams (PUs) and expanded polystyrene (EPS) that are versatile heat-insulation materials, or vacuum insulated panels (VIPs). There are almost no changes in heat-insulation performance of silica aerogels across the ages. Furthermore, silica aerogels have heat resistance of 400° C. or higher. For these reasons, silica aerogels have attracted a great deal of attention as next-generation heat-insulation materials.
PUs and EPS have problems of deterioration of heat-insulation performance due to extraction of gases over time, and poor heat resistance. VIPs have excellent heat-insulation performance of several milliWatts per milliKelvin. However, VIPs have problems such as aged deterioration resulting in loss of vacuum, and low heat resistance of only about 100° C.
Silica aerogels are superior to any other existing heat-insulation materials in terms of deterioration with age and heat resistance. Silica aerogels have excellent heat conductivities of around 15 mW/mK. However, silica aerogels have network structures in which silica particles on the scale of several tens of nanometers are connected in rows through point contact. Accordingly, silica aerogels do not have sufficient mechanical strength. Therefore, in order to overcome the weakness, studies have been made on methods for improving the strength by way of combining silica aerogels with fibers, unwoven fabrics, resins, etc.
In general, inorganic nanoporous materials such as silica aerogels are synthesized by a sol-gel method that is based on a liquid-phase reaction. Water glass (an aqueous solution of sodium silicate) or alkoxysilane compounds such as tetramethoxysilane are used as raw materials. These materials, and a liquid medium such as water or alcohols, and a catalyst, as needed, are mixed, and the materials are hydrolyzed. That is, the sol materials are subjected to a polycondensation in the liquid medium to convert the sol materials into a gel.
Then, the gel is caused to grow. This step is called aging. Aging is a step for causing the polycondensation reaction of the gel to proceed, thus reinforcing the skeletons.
Then, a silylation step is carried out. In this step, the gel is hydrophobized with a silylating agent. In cases where hydrophobization treatments are not carried out, contraction of gel skeletons due to strong capillary force will occur when the liquid medium in the gel is evaporated to dry the gel. Consequently, silica particles come into physical contact with each other, a dehydration condensation of silanols present on the surfaces of silica particles proceeds, and contraction and densification are induced. As a result, aerogels having inferior heat-insulation performance will be produced.
On the other hand, in cases where a hydrophobization treatment is carried out, silanols present on surfaces of silica particles in the gel sufficiently react with the silylating agent, and thus, the hydroxyl groups are capped. For this reason, even if the gel skeleton temporarily contracts due to the capillary force when the liquid medium inside the gel is evaporated to dry the gel, the contraction will be alleviated since any silanols are not present. As a result, contraction and densification of the aerogel will be suppressed, and thus, an aerogel having excellent heat-insulation performance will be produced.
This phenomenon is called springback. That is, the hydrophobization step is indispensable in order to cause this springback. In addition, it is very important to optimize an amount of a silylating agent included therein, a reaction system, temperature, time, etc. in the hydrophobization step.
Finally, the liquid medium inside the gel is evaporated to dry the gel. For a drying technique therefor, the supercritical drying method or non-supercritical drying method (ordinary-pressure drying methods, freeze-drying methods, etc.) can be used. The above-described synthesis is disclosed in WO/2007/010949, Japanese Patent No. 3,854,645, etc.